This patent specification relates to optical amplifiers. More specifically, it relates to a semiconductor optical amplifier capable of using transverse lasing to excite its gain medium.
As the world""s need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are capable of very high-bandwidth operation.
Optical amplifiers are important components of optical communications links. Optical amplifiers are commonly used as (i) power amplifiers at the source end of an optical communications link, (ii) line amplifiers along the optical signal transmission path, and (iii) preamplifiers at the receiving end of the optical communications link, and have other uses as well.
In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. However, EDFAs are quite bulky, having a typical fiber length of about 30 feet, and require the presence of a separate pumping laser to operate. Accordingly, EDFAs are difficult to incorporate into confined spaces, and are not amenable to circuit-board-level or chip-level integration.
SOAs, on the other hand, are small in size and conveniently integrated into small devices. An SOA generally resembles a semiconductor laser structure, except that the end mirrors have been replaced by antireflection coatings. In such devices the product of the gain and the reflectivity is less than one so that the device does not oscillate. Rather, the device is used to amplify an incoming optical signal as it passes through the device. Such devices are often called traveling wave amplifiers, which highlights the fact that the optical signal does not pass back and forth within the device, but merely passes through it essentially only once. SOAs generally yield lower output power and higher noise levels as compared to EDFAs, and/or are restricted to lower data rates. Research continues toward improving the performance of SOAs, including making SOAs with higher power and lower noise characteristics, and/or that are capable of operating at higher data rates.
Crosstalk is one of the primary troublesome noise sources in conventional SOAs, with amplified spontaneous emission (ASE) being the other primary troublesome noise source. Crosstalk, or cross-channel modulation, involves data-dependent gain fluctuations at high output levels from the SOA, and can occur for either time-multiplexed or wavelength-multiplexed data. Crosstalk arises from gain saturation effects in an SOA. These effects can be understood by recalling that SOA devices rely on the phenomenon of stimulated emission to provide the necessary amplification. Stimulated emission, in turn, requires the establishment of a population inversion. In typical SOAs or lasers a population inversion is evidenced by the presence of a specified carrier density. When a sufficiently large optical signal is passed through the amplifier, the population inversion is substantially reduced or depleted, i.e. the gain of the SOA is saturated, and is reestablished only over some finite period of time. Consequently, the gain of the SOA will be reduced for some period of time following the passage of the signal through the amplifier, a time period commonly denoted as the amplifier gain recovery time.
When the gain medium becomes saturated due to a high signal level on a first channel, changes are induced in the signal level of a second channel because the saturated gain medium cannot properly amplify both channels. Since gain is modulated by the first signal, this modulated gain is impressed on the second signal. Thus, for wavelength division multiplexed (WDM) systems in which a plurality of channels at xcex1, xcex2, . . . , xcexN are present in a common optical signal, gain saturation induced by a first channel at xcex1 can produce unwanted level changes (i.e., errors) in a second data channel at xcex2, and vice versa.
Crosstalk can be reduced by keeping the SOA out of gain saturation for the data rates, signal levels, and number of channels on the optical signal of interest. If the SOA is operated near gain saturation levels, crosstalk may be reduced by making the period of the data signals small in comparison to the gain recovery time, i.e., by slowing down the data rate. In general, an SOA will have reduced crosstalk effects if (i) its saturation power PSAT, i.e., the input optical power level for which the SOA gain is reduced to a predetermined percentage of its nominal value, is increased, and/or (ii) its gain recovery time is decreased. As used herein, an SOA has increased gain stability if (i) its saturation power PSAT is increased without a concomitant increase in gain recovery time, (ii) its gain recovery time is decreased without a concomitant decrease in saturation power PSAT, or (iii) both (i) and (ii) occur.
Several methods for dealing with crosstalk problems are discussed in U.S. Pat. No. 5,436,759, which is incorporated by reference herein. One strategy is to place a transverse laser across the SOA such that the laser""s gain medium and the SOA""s signal gain medium share an overlapping region. The lasing cavity is operated above threshold and the gain of the laser is clamped to overcome losses of the cavity. As used herein, a laser cavity is gain-clamped and lasing when it is excited by a bias current greater than a threshold current. When the transverse laser is gain-clamped, gain along the SOA signal path is stabilized. The transverse lasing enhances the establishment and maintenance of a population inversion in the overlapping region, resulting in both increased saturation power and a decreased gain recovery time. Advantageously, independent lasing only builds up in the transverse direction and does not corrupt the quality of the amplified signal.
The ""759 patent supra discusses an SOA in which an input optical signal is amplified by a signal gain medium along a signal path, the signal path being intersected by a segmented optical cavity oriented off-axis (e.g., perpendicular) to the signal path. The optical cavity is a lasing cavity operated above threshold, and shares its gain medium with the signal gain medium at overlapping locations, thereby increasing gain stability. Certain segmentation and design techniques are proposed for dealing with parasitic lasing modes that can cause gain clamping at undesirably low levels, with some designs directed to suppressing the parasitic lasing modes (e.g., ""759 patent, FIG. 1), and other designs directed to constructively using circulating modes (e.g., ""759 patent, FIG. 2B) to increase the gain. To suppress parasitic lasing modes, the lasing cavity is segmented along the length of the amplifier with regions that are optically isolated, except at intersections with the gain medium/signal path. In some examples, the optical isolation is achieved by placing gaps between the cavities that include opaque barriers (""759 patent, FIG. 1), while in other examples angled trenches are used (""759 patent, FIG. 2C).
The proposed designs of the ""759 patent supra can suffer from one or more shortcomings that can reduce the effectiveness of the device and/or cause difficulty in reliably fabricating the device. For example, the layers of the single active medium (""759 patent, FIG. 1) can be difficult to construct reliably with consistent thickness across the entire lateral area to be covered. In addition, the semiconductor layers may contain local defects such as crystal dislocations, pitting, voids, etc. Such defects in the epitaxial growth can be a point of lower electrical resistance than the surrounding epitaxial areas. The higher electrical current flowing through these points of lower electrical resistance can create xe2x80x9chot spotsxe2x80x9d which cause non-uniform gain in the effected areas. In addition to the less than optimal performance resulting form the non-uniform gain, the xe2x80x9chot spotsxe2x80x9d can be a source of excessive current drain and premature device failure. As another example, undesired parasitic modes or uneven lasing may arise due to lack of electrical isolation among laser cavity segments at intersections with the signal gain medium.
Another approach to gain saturation reduction is discussed in Francis, et. al., xe2x80x9cA Single Chip Linear Optical Amplifier,xe2x80x9d IEEE Optical Fiber Communication Conference, Anaheim, Calif. (2001), which is incorporated by reference herein, in which the amplifier and a vertical cavity surface emitting laser (VCSEL) share the same active region. However, these devices can have limited output power due to the small gain medium volume of VCSELs, as well as for other reasons. These devices can also suffer from non-uniform current flows and hot spots due to defects in crystal growth and non-uniform epitaxial layers.
An SOA experiencing crosstalk and other gain saturation effects is generally operating in a nonlinear region of operation, in which the signal gain is not a constant value over time. Such an SOA is generally not desirable for use as an amplifier in an optical communications link. However, devices with nonlinear input-output characteristics are increasingly finding use in other applications, including all-optical gating applications, wavelength conversion applications, and all-optical signal regeneration applications.
Accordingly, it would be desirable to provide a semiconductor optical amplifier having reduced crosstalk effects.
It would be further desirable to provide a semiconductor optical amplifier that can have an adjustable gain along its length, such that the gain can be adjusted at the factory and/or in the field during operation.
It would be still further desirable to provide a semiconductor optical amplifier that can optionally be adjusted to have nonlinear gain characteristics along one or more segments of the signal path, while having linear gain characteristics along other segments of the signal path.
It would be even further desirable to provide such a semiconductor optical amplifier in which amplified spontaneous emission (ASE) noise is reduced in the output.
It would be even further desirable to provide such a semiconductor optical amplifier that is more robust against variations in gain medium thickness and/or other crystal defects.
It would be still further desirable to provide such a semiconductor optical amplifier in which the operating current requirements are kept low.
It would be even further desirable to provide such a semiconductor optical amplifier that can be reliably fabricated.
According to a preferred embodiment, a semiconductor optical amplifier (SOA) apparatus and related methods are provided for amplifying an optical signal, the SOA comprising a signal waveguide for guiding the optical signal along a signal path, the SOA further comprising a gain medium that is excited by the lasing fields of two or more transverse laser cavities intersecting the signal waveguide along the signal path. The transverse laser cavities are biased above a threshold current to achieve gain-clamped operation, wherein gain stability along the signal path is increased and crosstalk in the amplified optical signal is reduced. According to a preferred embodiment, the transverse laser cavities are non-overlapping, each transverse laser cavity intersecting the signal waveguide at its own distinct region of intersection.
As the optical signal propagates down the signal waveguide, it encounters the regions of intersection and is amplified by the excited gain medium therein. Between the regions of intersection along the signal path, the optical signal encounters areas of reduced amplification or loss, termed connecting zones. According to a preferred embodiment, the connecting zones comprise at least one portion of wave guiding material having an increased electrical resistivity as compared to corresponding wave guiding material in the regions of intersection. This provides a measure of electrical isolation between adjacent transverse laser cavities as they intersect the signal waveguide, reducing unwanted parasitic lasing modes among the transverse laser cavities.
The connecting zones may be maintained at or near a condition of transparency while having little or no gain, such that gain guiding in the transverse laser cavities is facilitated. It has been found that several advantages associated with the increased-resistance connecting zones, e.g., electrical isolation of transverse lasing cavities for increased SOA gain profile control in terms of both amplitude and wavelength, reduction of parasitic lasing modes, etc., can justify any reduction in amplification caused by the increased-resistance connecting zones along the signal path, provided that the overall gain is sufficient. According to a preferred embodiment, the percentage of the signal path occupied by the connecting zones is substantially less than the percentage of the signal path occupied by the regions of intersection of the transverse laser cavities such that positive-gain operation is achieved.
According to a preferred embodiment, the transverse laser cavities are electrically separated along their lengths (in the propagation direction of the amplified signal) by proton-implanted isolation regions, which include the connecting zones. Optionally, the degree of electrical isolation in the connecting zones may be different than that in the remainder of the isolation regions. According to a preferred embodiment, the gain medium lying in the connecting zones is disordered or partially disordered, thereby reducing signal losses in the connecting zones. In contrast, in areas corresponding to the remainder of the isolation regions, the gain medium is not disordered. Because there is essentially no current in these areas, optical isolation between adjacent transverse laser cavities is provided in these areas in addition to electrical isolation. In an alternative preferred embodiment, all of the gain medium in the isolation regions is disordered. In this alternative preferred embodiment, the electrical isolation alone between adjacent transverse laser cavities is sufficient to achieve operational segregation and reduction of unwanted parasitic lasing modes.
A variety of geometric layouts for the transverse laser cavities may be used in accordance with the preferred embodiments. The transverse laser cavities may be substantially perpendicular to the signal waveguide, or may alternatively be at a different non-perpendicular angle. In one preferred embodiment, the transverse laser cavities are oriented at the Brewster angle with respect to the signal waveguide for reducing reflections at the interfaces of dissimilar materials. In another preferred embodiment, the transverse laser cavities are optically connected with mirrors to form a single folded-path laser with multiple crossings of the signal waveguide.
According to a preferred embodiment, the transverse laser cavities are provided with separate electrical contacts so that they may be biased at different currents and current density levels. The separate control of the current to different transverse laser cavities allows for compensation for local hot spots caused by crystal dislocations, voids, or other local defects in the semiconductor material. Different groupings or couplings may be made among the electrical contacts of the transverse laser cavities. In another preferred embodiment, the SOA can be operated as a variable optical attenuator (VOA) by variably reducing the bias currents of one or more transverse lasers, such that the collective gain at the regions of intersection is overcome by the collective loss at the connecting zones.
According to another preferred embodiment, an SOA is provided comprising a signal waveguide for guiding an optical signal along a signal path, the SOA further comprising a gain medium that is excited by the lasing field of one or more multi-contact transverse laser cavities intersecting the signal waveguide along the signal path. Each multi-contact transverse laser cavity comprises two end mirrors defining a laser cavity therebetween, the laser cavity being segmented along the direction of the lasing field into a center segment and at least one end segment, the end segments and the center segment each having their own current source for biasing. In a first configuration, the end segments are provided with no bias current, thereby impeding gain clamping in the laser cavity and causing gain nonlinearities along the signal path. In a second configuration, the end segments are provided with bias currents sufficient to facilitate gain clamping in the laser cavity, thereby allowing linear gain along the signal path. Accordingly, depending on the amount bias current through the end segments, which may be dynamically controlled, the multi-contact transverse laser cavity may effectuate linear or nonlinear gain along its respective portion of the signal path. In a preferred embodiment, a plurality of multi-contact transverse laser cavities intersect the signal path including first and second subsets thereof, the first subset operating in linear-gain mode and the second subset operating in nonlinear-gain mode.
According to another preferred embodiment, amplified spontaneous emission (ASE) noise in the output can be reduced by including a second signal waveguide resonantly coupled with a first signal waveguide. The optical signal is amplified by the first signal waveguide in a gain-stabilized manner by a gain medium excited by one or more transverse lasing cavities intersecting the first signal waveguide. The optical signal is then transferred into the second signal waveguide prior to output. The ASE generated along the first signal waveguide does not couple effectively into the second signal waveguide, and ASE in the output is thereby reduced. Optionally, the second signal waveguide may also provide some amplification of the optical signal. Optionally, tunable coupling between the waveguides can be provided such that the degree of coupling can be controlled by electrical, optical, mechanical, or other signals.